In implementing LED light sources, arrangements are conventionally resorted to which comprise plural LED “strings”, which are fed by one and the same supply source.
Strings may differ from one another in various respects, for example in the number and kind of LEDs, in the operating temperatures and other parameters, so that voltage across a string can be different from the voltage across the other string(s).
For this reason, a solution of directly connecting in parallel strings with one another turns out not to be viable (even when an ideal or quasi-ideal current generator is used as a supply source), because the supply power is ultimately distributed to the various strings in an uncontrolled fashion.
The diagrams and FIGS. 1 to 3 show various solutions that can be used to ensure a better uniformity in power distribution on plural LED strings, denoted in general by references K1, K2, . . . , Kn, wherein n can virtually be any number higher than one.
In the diagrams of FIGS. 1 to 3 (as in the other Figures annexed to the present disclosure), the supply generator is shown ideally as in parallel between an ideal current generator, adapted to generate a current I, and a capacitor CI.
The three diagrams of FIGS. 1 to 3 have a current regulator associated to each string K1, K2, . . . , Kn.
This can be achieved, for instance:                by simply resorting to a resistor R1, R2, . . . , Rn, as shown in FIG. 1,        in the form of an active linear regulator (for example a bipolar transistor Q1, Q2, . . . Qn), as shown in FIG. 2,        by using more complex switching regulators, for example in the form of buck converters comprising, for each string K1, K2, . . . , Kn, an inductor L1, L2, . . . , Ln and a switch Q1, Q2, . . . Qn (e.g. a mosfet) adapted to be traversed by the current flowing in the LED string K1, K2, . . . , Kn, as well as a freewheeling diode D1, D2, . . . , Dn, as shown in FIG. 3.        
In the latter arrangement there is moreover provided a current measure and control circuit (denoted in FIG. 3 as CMC, i.e. Current Measure and Control) which, on the basis of the intensity of the current traversing the various strings K1, K2, . . . , Kn, as detected via sensors or probes P1, P2, . . . , Pn (of any known kind) performs a corresponding function of current control in the various strings K1, K2, . . . , Kn, by opening and closing Q1, Q2, . . . , Qn according to need.
The exemplary solutions shown in the diagrams of FIGS. 1, 2 and 3 suffer from various drawbacks.
Specifically, the solutions implementing a linear control function (see FIGS. 1 and 2), if on one hand are easy to implement, have the intrinsic disadvantage of causing a power dissipation which is proportional to the operating voltage difference of the various strings K1, K2, . . . , Kn and to the work current of such strings, such power being completely lost. A solution as shown in FIG. 1 has moreover the drawback of needing a virtually fixed compensation mechanism.
Switching solutions such as shown in FIG. 3 involve the presence of an additional “intelligence”, in order to identify which sets of the various switches Q1, Q2, . . . , Qn must be kept closed at any time and which ones must be kept opened, in order to perform the balancing function needed, according to the control requirements provided by the CMC module. Moreover, in solutions as shown in FIG. 3, each regulator must be able to manage all the power involved in the operation of the string to which the switch is coupled.
Solutions which substantially derive from the current mirror arrangement of FIG. 2 are described in documents such as U.S. Pat. No. 7,317,287 or U.S. Pat. No. 6,621,235.
The state of the art comprises moreover document WO-A-2010/000333 (which substantially reproduces the arrangement in FIG. 2, i.e. the use of analogically driven transistors).
To complete the survey we refer to the solution disclosed in document US-A-2010/0315013, which is based on the use of a switching converter, which can be broadly defined as a series/parallel converter typically comprising a transformer for each string.